A conventional mixer circuit (hereinafter referred to as image rejection mixer or IRM) for suppressing image interference is described.
FIG. 16 shows a conventional IRM. As shown in FIG. 16, the conventional IRM comprises input terminal 1101 to which high-frequency signal is inputted; mixer 1103 of which the high-frequency signal inputted to the input terminal 1101 is supplied to one input and the output signal of oscillator 1102 is supplied to the other input; output terminal 1107 to which the output from the mixer 1103 is supplied; mixer 1105 of which the high-frequency signal inputted to the input terminal 1101 is supplied to one input; oscillator phase shifter 1104 which is connected between the other input of the mixer 1105 and the output of the oscillator 1102 and also serves to shift the phase of the other input of the mixer 1103 by −π/2 radian; intermediate frequency phase sifter 1106 which is connected between the output of the mixer 1105 and the output terminal 1107 and serves to shift the output of the mixer 1105 by −3π/2 radian in phase.
The operation of the mixer circuit configured as described above is set so that the phase difference at the output of the mixer 1105 becomes −π/2 radian as against the phase at the output of the mixer 1103. Further, since the intermediate frequency phase shifter 1106 with phase shift of −3π/2 raidan is inserted in the output of the mixer 1105, it is possible to allow the desired signal pass and to cancel the image interfering signal.
The detailed description is given in the following. Here, all of the amplitudes of the input signal, fundamental output signal of oscillator 1102, and tertiary higher harmonic components generated by limiter circuits in the mixer 1103 and 1105 receiving the fundamental output component from the oscillator 1102 are 1. Also, all of the gain of mixer 1103 and 1105, the gain of oscillator phase shifter 1104, and the gain of intermediate frequency phase shifter 1106 are 1, simplifying the following description.
The reason for simplifying the description is such that the phase is a problem of greater concern rather than the amplitude because signals from the output terminal 1107 are outputted with two signals added or suppressed.
Specifically, the amplitude is 1 with respect to each of desired signal Vd being the input signal, image interfering signal Vi, interfering signal (hereafter Vm1) being at a lower side apart by IF (frequency of intermediate frequency signal) as against a frequency three times higher than the fundamental frequency from the oscillator 1102, and interfering signal (hereafter Vm2) being at an upper side apart by IF as against a frequency three times higher than the fundamental frequency from the oscillator 1102.
Also, the amplitude of fundamental output component VL of the oscillator 1102 is 1. The amplitude is 1 with respect to each of tertiary higher harmonic component VL3a generated by the limiter circuit in the mixer 1103 receiving the fundamental component VL from the oscillator 1102 and tertiary higher harmonic component VL3b generated by the limiter circuit in the mixer 1105 receiving the fundamental output component from the oscillator 1102 via the oscillator phase shifter 1104. The tertiary higher harmonic component VL3a and tertiary higher harmonic component VL3b are actually lower in amplitude as against the fundamental output component VL of the oscillator 1102, but the amplitude is to be 1 for simplifying the description.
Firstly, the description will be given by using calculating expressions with respect to a case such that desired signal Vd and image interfering signal Vi are inputted to the input terminal 1101.
Desired signal Vd is represented by formula 1. Here, angular frequency is ω1, time is t, and phase angle is −θ1.Vd=sin(ω1t−θ1)  (formula 1)
Also, image interfering signal Vi is represented by formula 2. Here, angular frequency is ω3, and phase angle is −θ3.Vi=sin(ω3t−θ3)  (formula 2)
Further, fundamental output component VL of the oscillator 1102 is represented by formula 3. Here, angular frequency is ω2, and phase angle is −θ2.VL=sin(ω2t−θ2)  (formula 3)
The desired signal Vd and image interfering signal Vi are respectively divided into two signals to be inputted to one input 1103a of the mixer 1103, and in case the fundamental output component VL of the oscillator 1102 is inputted to the other terminal 103b of the mixer 1103, signal V (1103c) outputted from the output 1103c of the mixer 1103 is a signal represented by formula 4.
                                                                        V                ⁡                                  (                                      1103                    ⁢                    c                                    )                                            =                            ⁢                                                1                  /                  2                                ×                                  (                                      Vd                    +                    Vi                                    )                                ×                VL                                                                                        =                            ⁢                                                1                  /                  2                                ×                                  (                                                                                    1                        /                        2                                            ×                                              cos                        ⁡                                                  (                                                                                                                    ω                                2                                                            ⁢                              t                                                        -                                                                                          ω                                1                                                            ⁢                              t                                                        +                                                          θ                              1                                                        -                                                          θ                              2                                                                                )                                                                                      +                                                                                                                                        ⁢                                                1                  /                  2                                ×                                  cos                  ⁡                                      (                                                                                            ω                          3                                                ⁢                        t                                            -                                                                        ω                          2                                                ⁢                        t                                            -                                              θ                        3                                            +                                              θ                        2                                                              )                                                              )                                                          (                  formula          ⁢                                          ⁢          4                )            
Next, same signal as that of the mixer 1103 is inputted to one input 1105a of the mixer 1105. A signal of which the phase of oscillator 1102 is shifted by −π/2 radian by the oscillator phase shifter 1104 is inputted to the other input 1105b of the mixer 1105. Accordingly, signal V (1105c) that is IF signal component (intermediate frequency component) from output 1105c of the mixer 1105 can be represented by formula 5.
                                                                        V                ⁡                                  (                                      1105                    ⁢                    c                                    )                                            =                            ⁢                                                1                  /                  2                                ×                                  (                                      Vd                    +                    Vi                                    )                                ×                                  sin                  ⁡                                      (                                                                                            ω                          2                                                ⁢                        t                                            -                                              θ                        2                                            -                                              π                        /                        2                                                              )                                                                                                                          =                            ⁢                                                1                  /                  2                                ×                                  (                                                            1                      /                      2                                        ×                                                                                                                                        ⁢                                                cos                  ⁢                                      (                                                                                            ω                          2                                                ⁢                        t                                            -                                                                        ω                          1                                                ⁢                        t                                            +                                              θ                        1                                            -                                              θ                        2                                            -                                              π                        /                        2                                                              )                                                  +                                                                                                      ⁢                                                1                  /                  2                                ×                                  cos                  ⁡                                      (                                                                                            ω                          3                                                ⁢                        t                                            -                                                                        ω                          2                                                ⁢                        t                                            -                                              θ                        3                                            +                                              θ                        2                                            +                                              π                        /                        2                                                              )                                                              )                                                          (                  formula          ⁢                                          ⁢          5                )            
Since the signal V (1105c) is phase-shifted by −3π/2 radian by the intermediate frequency phase shifter 1106, the signal V (1106a) of the output 1106a of the intermediate frequency phase shifter 1106 can be represented by formula 6.
                                                                        V                ⁡                                  (                                      1106                    ⁢                    a                                    )                                            =                            ⁢                                                1                  /                  2                                ×                                  (                                                            1                      /                      2                                        ×                                          cos                      ⁡                                              (                                                                                                            ω                              2                                                        ⁢                            t                                                    -                                                                                    ω                              1                                                        ⁢                            t                                                    +                                                      θ                            1                                                    -                                                      θ                            2                                                                          )                                                                                                                                                                                    =                                ⁢                                                                            +                      1                                        /                    2                                    ×                                      cos                    ⁡                                          (                                                                                                    ω                            3                                                    ⁢                          t                                                -                                                                              ω                            2                                                    ⁢                          t                                                -                                                  θ                          3                                                +                                                  θ                          2                                                -                        π                                            )                                                                                  )                                                          (                  formula          ⁢                                          ⁢          6                )            
Also, since V (1103c) and V (1106a) are compounded at the output terminal 107, the IF signal component of V (1107) represented by formula 7 is outputted from the output terminal 1107.
                                                                        V                ⁡                                  (                  1107                  )                                            =                            ⁢                                                V                  ⁡                                      (                                          1103                      ⁢                      c                                        )                                                  +                                  V                  ⁡                                      (                                          1106                      ⁢                      a                                        )                                                                                                                          =                            ⁢                                                1                  /                  2                                ×                                  cos                  ⁡                                      (                                                                                            ω                          2                                                ⁢                        t                                            -                                                                        ω                          1                                                ⁢                        t                                            +                                              θ                        1                                            -                                              θ                        2                                                              )                                                                                                          (                  formula          ⁢                                          ⁢          7                )            
As is obvious in formula 7, the first term of V (1103c) and the first term of V (1106a) which are the IF components of desired signal Vd are compounded twofold because of being coincident in phase with each other. Consequently, the IF component of desired signal Vd shown in formula 7 is outputted from the output terminal 1107.
On the other hand, the second term of V (1103c) and the second term of V (1106a) which are the IF components of image interfering signal Vi are canceled because of being in π radian relation in phase with each other. Consequently, the IF component of image interfering signal Vi is not outputted from the output terminal 1107.
FIG. 17 is a chart showing the phases of the desired signal and image interfering signal in the above conventional mixer circuit.
FIG. 17 shows the phases with respect to the desired signal and image interfering signal in the mixer 1103 and the mixer 1105, and their amplitudes are not represented. Further, the desired signal is shown by solid lines, and the image interfering signal is shown by dotted lines.
Also, for simplifying the description, all of the respective phases −θ2, −θ1 and −θ3 of the fundamental output component of the oscillator 1102, the desired signal and the image signal supplied from the input terminal 1101 are 0 radian in the following description.
Firstly, the desired signal Vd inputted to one input 1103a of the mixer 1103 is signal 1131, and the image interfering signal Vi is signal 1132, and the phases of both signals are 0 radian in the representation. Next, signal 1133 inputted to the other input 1103b of the mixer 1103 can be same in phase in the representation. Further, from the formula 4, the desired signal 1134 outputted as IF signal component from the output 1103c of the mixer 1103 becomes 0 radian in phase, and the image interfering signal 1135 outputted as IF signal component also becomes 0 radian in phase.
Secondly, the desired signal Vd to one input 1105a of the mixer 1105 is signal 1136, and the image interfering signal Vi is signal 1137, same as the signal 1131 of desired signal Vd inputted to the mixer 1103 and the signal 1132 of image interfering signal Vi. Next, since the signal 1138 is inputted to the other input 1105b of the mixer 1105 via the oscillator phase shifter 1104, it is represented as a signal phase-shifted by −π/2 radian as against the output signal of oscillator 1102.
As shown in the formula 5, the desired signal 1139 outputted as IF signal component from the output 1105c of the mixer 1105 is phase-shifted by −π/2 radian. Also, the image interfering signal 1140 outputted as IF signal component from the output 1105c of the mixer 1105 is phase-shifted by π/2 radian. Further, since the output 1106a is phase-shifted by −3π/2 radian by the intermediate frequency phase shifter 1106, the desired signal 1141 and the image interfering signal 1142 can be represented as shown in FIG. 17.
Finally, the desired signal outputted as IF signal component from the output terminal 1107 is a composite signal of the desired signal 1134 outputted from the mixer 1103 and the desired signal 1141 outputted from the mixer 1105. These desired signals 1134 and 1141 being coincident in phase with each other are doubled to become the desired signal 1143. Also, the image interfering signal outputted as IF signal component from the output terminal 1107 is a composite signal of the image interfering signal 1135 outputted from the mixer 1103 and the image interfering signal 1142 outputted from the mixer 1105. In this case, the image interfering signals 1135 and 1142 are in π radian relation in phase difference and canceled to become 0. Accordingly, no image interfering signal is outputted. This coincides with the aforementioned formula 7.
As described above, the desired signal is allowed to pass, but the image interfering signal is canceled and not outputted.
A conventional mixer circuit that may prevent image interference is disclosed in Japanese Laid-open Patent S52-132710.
Also, a like circuit is mentioned in “RF MICROELECTRONICS” written by Behzad Razavi (University of California, Los Angeles), P138–P146, 1998.
However, in such conventional configuration, for example, when TV broadcasting signal of relatively low frequency channel is received, it is possible to suppress the image interfering signal. However, in case of interfering signal being at a lower or upper side apart by intermediate frequency (hereafter called IF) as against a frequency three times higher than the fundamental frequency from the oscillator 1102, it will cause the generation of spurious, and the signal reception is disturbed or it becomes unable to receive the signal.
The reasons for the above are mentioned in the following. Here, it is supposed that interfering signal being at a lower or upper side apart by IF as against a frequency three times higher than the fundamental frequency from the oscillator 1102 is inputted to the input terminal 1101, and the description will be given by using calculating expressions.
Interfering signal Vm1 being at a lower side apart by IF as against a frequency three times higher than the fundamental frequency from the oscillator 1102 is represented by formula 8. Here, the angular frequency is ω4, and the phase is −θ4.Vm1=sin(ω4t−θ4)  (formula 8)
Further, interfering signal Vm2 being at an upper side apart by IF as against a frequency three times higher than the fundamental frequency from the oscillator 1102 is represented by formula 9. Here, the angular frequency is ω5, and the phase is −θ5.Vm2=sin(ω5t−θ5)  (formula 9)
VL3a that is the tertiary higher harmonic component generated by a limiter circuit in the mixer 1103 receiving the fundamental output component from the oscillator 1102 can be represented by formula 10.VL3a=sin(3ω2t−3θ2)  (formula 10)
VL3b that is the tertiary higher harmonic component generated by a limiter circuit in the mixer 1105 receiving the fundamental output component from the oscillator 1102 via the oscillator phase shifter 1104 can be represented by formula 11.VL3b=sin(3ω2t−3θ2−3π/2)  (formula 11)
Interfering signals Vm1 and Vm2 are respectively divided into two signals and inputted to one terminal 1103a of the mixer 1103. The equivalent of this is that tertiary higher harmonic component VL3a is inputted to the other input 1103b of the mixer 1103. Therefore, V (1103c) outputted as IF component from the output 1103c of the mixer 1103 is a signal that can be represented by formula 12.
                                                                        V                ⁡                                  (                                      1103                    ⁢                    c                                    )                                            =                            ⁢                                                1                  /                  2                                ×                                  (                                      Vm1                    +                    Vm2                                    )                                ×                VL3a                                                                                        =                            ⁢                                                1                  /                  2                                ×                                  (                                                            1                      /                      2                                        ×                                                                                                                                        ⁢                                                cos                  ⁡                                      (                                                                  3                        ⁢                                                  ω                          2                                                ⁢                        t                                            -                                                                        ω                          4                                                ⁢                        t                                            +                                              θ                        4                                            +                                              3                        ⁢                                                  θ                          2                                                                                      )                                                  +                                                                                                      ⁢                                                1                  /                  2                                ×                                  cos                  ⁡                                      (                                                                                            ω                          5                                                ⁢                        t                                            -                                              3                        ⁢                                                  ω                          2                                                ⁢                        t                                            +                                              θ                        5                                            +                                              3                        ⁢                                                  θ                          2                                                                                      )                                                              )                                                          (                  formula          ⁢                                          ⁢          12                )            
Next, same signal as that of the mixer 1103 is inputted to one input 1105a of the mixer 1105. The equivalent of this is that tertiary higher harmonic component VL3b is inputted to the other input 1105b of the mixer 1105. Therefore, V (1105c) outputted as IF component from the output 1105c of the mixer 1105 is a signal that can be represented by formula 13.
                                                                        V                ⁡                                  (                                      1105                    ⁢                    c                                    )                                            =                            ⁢                                                1                  /                  2                                ×                                  (                                      Vm1                    +                    Vm2                                    )                                ×                VL3b                                                                                        =                            ⁢                                                1                  /                  2                                ×                                  (                                                            1                      /                      2                                        ×                                        ⁢                                          cos                      (                                                                        3                          ⁢                                                      ω                            2                                                    ⁢                          t                                                -                                                                              ω                            4                                                    ⁢                          t                                                +                                                  θ                          4                                                -                                                                                                                                                                                                    ⁢                                                      3                    ⁢                                          θ                      2                                                        -                                      3                    ⁢                                          π                      /                      2                                                                      )                            +                                                1                  /                  2                                ×                                  cos                  (                                                                                    ω                        5                                            ⁢                      t                                        -                                          3                      ⁢                                              ω                        2                                            ⁢                      t                                        -                                                                                                                                                          ⁢                                                      θ                    5                                    +                                      3                    ⁢                                          θ                      2                                                        +                                      3                    ⁢                                          π                      /                      2                                                                      )                            )                                                          (                  formula          ⁢                                          ⁢          13                )            
Since the V (1105c) is phase-shifted by −3π/2 radian by the oscillator phase shifter 1106, V (1106a) that is the output 1106a of the oscillator phase shifter 1106 can be represented by formula 14.V(1106a)=½×(½×cos(3ω2t−ω4t+θ4−3θ2−π)+½×cos(ω5t−3ω2t−θ5+3θ2))  (formula 14)
Also, V (1107) outputted from the output terminal 1107 is a composite signal of V (1103c) and V (1106a), and can be represented by formula 15.
                                                                        V                ⁡                                  (                  1107                  )                                            =                            ⁢                                                V                  ⁡                                      (                                          1103                      ⁢                      c                                        )                                                  +                                  (                  V1106a                  )                                                                                                                        =                                ⁢                                                      1                    /                    2                                    ×                                      cos                    ⁡                                          (                                                                                                    ω                            5                                                    ⁢                          t                                                -                                                  3                          ⁢                                                      ω                            2                                                    ⁢                          t                                                -                                                  θ                          5                                                +                                                  3                          ⁢                                                      θ                            2                                                                                              )                                                                                  )                                                          (                  formula          ⁢                                          ⁢          15                )            
As is obvious in formula 15, the first term of V (1103c) and the first term of V (1106a) which are the IF components of interfering signal Vm1 are in π radian relation in phase with each other and canceled. Consequently, the IF component of interfering signal Vm1 is not outputted from the output terminal 1107.
On the other hand, the second term of V (1103c) and the second term of V (1106a) which are the IF components of interfering signal Vm2 are coincident in phase with each other and compounded twofold to be outputted from the output terminal 1107. That is, interfering signal Vm2 being at an upper side apart by IF as against a frequency three times the fundamental frequency from the mixer 1102 is outputted as an interfering signal.
FIG. 18 is a diagram showing the phases of interfering signals related to a frequency three times higher than the fundamental frequency from the oscillator in a conventional mixer circuit.
FIG. 18 shows the phases of interfering signals being at a lower or upper side apart by IF as against a frequency three times higher than the fundamental frequency from the oscillator 1102 at each portion of the mixer 1103 and mixer 1105, and includes no representation about the amplitudes.
In this case, interfering signal Vm1 is shown by solid lines, and interfering signal Vm2 is shown by dotted lines.
Firstly, interfering signals Vm1 and Vm2 are signal 1171 and signal 1172 respectively. The signal 1171, interfering signal Vm1, and the signal 1172, interfering signal Vm2, are inputted to one terminal 1103a of the mixer 1103. Also, tertiary higher harmonic component VL3a generated in the mixer 1103 can be represented by signal 1173 from the formula 10. Further, the IF components of interfering signals Vm1 and Vm2 outputted from the output 1103c of the mixer 1103, as is obvious in the formula 12, can be represented by signal 1174 and signal 1175 respectively.
Next, the interfering signals Vm1 and Vm2 inputted to one input 1105a of the mixer 1105 are represented by signal 1176 and signal 1177 respectively. Here, the signal 1176 is inputted as same signal as signal 1171, and the signal 1177 is inputted as same signal as signal 1172.
Also, the fundamental output component of the oscillator 1102 is phase-shifted by −π/2 radian by the oscillator phase shifter 1104 before being inputted to the other input of the mixer 1105. Accordingly, VL3b that is the tertiary higher harmonic component generated by the limiter circuit in the mixer 1105 can be represented by signal 1178 from the formula 11. Further, the IF components of interfering signals Vm1 and Vm2 outputted from the output 1105c of the mixer 1105 can be respectively represented by signal 1179 and signal 1180 from the formula 13.
Next, the IF components of interfering signals Vm1 and Vm2 from the output 1105c are phase-shifted by −3π/2 radian at the oscillator phase shifter 1106. Accordingly, at the output 1106a, the components become signal 1181 and signal 1182 respectively from the formula 14.
Thus, the IF component of interfering signal Vm1 outputted from the output terminal 1107 is a composite signal of signal 1174 and signal 1181. Since the signal 1174 and signal 1181 are in π radian relation in phase with each other, the interfering signal Vm1 is canceled and not outputted.
On the other hand, the IF component of interfering signal Vm2 outputted from the output terminal 1107 is a composite signal of signal 1175 and signal 1182. In this case, however, since the signals 1175 and 1182 are coincident in phase with each other, the signals are added to become signal 1183 and outputted. This coincides with the calculation result of the formula 15.
The output signal from the oscillator 1102 is inputted to the other inputs of the mixer 1103 and 1105, but the input amplitude from the oscillator 1102 for enhancing the fundamental frequency performance of the mixer 1103 and 1105 is sufficiently raised by amplifiers in the mixer 1103 and 1105 before using. Or, the input signal from the oscillator 1102 is changed to a rectangular wave signal by the limiter circuit. The rectangular wave signal is used for switching the received signal as the operation of a mixer.
Therefore, even when the higher harmonic component of the oscillator 1102 itself is of low amplitude, higher harmonic components are generated by the amplifiers or limiter circuits in the mixer 1103 and 1105 at the later stage. Incidentally, when the circuit is integrated, a circuit of differential type suited for integration is employed, and then, the higher harmonic component of even multiple is of relatively low amplitude, but the higher harmonic component of odd multiple becomes higher.
Also, since the amplitude of tertiary higher harmonic component or quinary higher harmonic component close to the fundamental output component of the oscillator 1102 is relatively high, if there exists an interfering signal in a frequency being at an upper side or lower side apart by IF as against the higher harmonic component, the signal reception is affected and it becomes unable to receive the signal because of interference.
Table 1 shows whether interfering signals related to higher harmonic components generated by the fundamental output component of the oscillator 1102 can be prevented or not at a conventional IRM.
TABLE 1As against n times ×As against n times ×(fundamental) − IF(fundamental) + IFInput frequencyEffectsInput frequencyEffectsn = 1FundamentalDesiredImage∘frequency − IFsignalinterferencen = 33 times × fundamental∘3 times ×xfrequency − IFfundamentalfrequency + IFn = 55 times × fundamentalx5 times ×∘frequency − IFfundamentalfrequency + IFn = 77 times × fundamental∘7 times ×xfrequency − IFfundamentalfrequency + IFn = 99 times × fundamentalx9 times ×∘frequency − IFfundamentalfrequency + IFn = 1111 times × fundamental∘11 times ×xfrequency − IFfundamentalfrequency + IFn = 1313 times × fundamentalx13 times ×∘frequency − IFfundamentalfrequency + IF...............∘ - effective to suppress, able to eliminate.x - not effective to suppress, unable to eliminate.
In Table 1, when “n” representing the multiple of the fundamental frequency of oscillator 1102 is 1, it shows the fundamental frequency as the oscillator 1102, and (fundamental−IF) stands for desired signal, and (fundamental+IF) stands for image interfering signal. It shows that it is effective to suppress the image interfering signal.
Further, it shows that it is effective to suppress interfering signal Vm1 but not effective to suppress interfering signal Vm2 when “n” is 3. Also, it shows that it is hardly effective to suppress the interfering signal being either at a lower side or at an upper side apart by IF as against a frequency n times higher when “n” of the fundamental frequency from the oscillator 1102 is 5 or over.
Thus, in TV broadcasting signal, when a channel of relatively low frequency is received, there arises a problem that the signal reception is affected by interfering signal in case the broadcasting channel is at a frequency being at an upper side apart by IF as against a frequency three times higher than the fundamental frequency from the oscillator 1102 or at a frequency being at a lower side apart by IF as against a frequency five times higher.